Ion channels in membranes.

The membranes that separate the myocardial cell interior from the extracellular space and delimit compartments within the myocardial cell represent ion-impermeable phospholipid barriers. Embedded in these phospholipid membranes are intrinsic membrane proteins, some of which serve as ion channels. The voltage-sensitive ion channels that control the sarcolemmal action potential appear to be highly regulated intrinsic membrane proteins that contain "gates" that respond to changing membrane potential by opening and closing an ion-selective "pore" that allows specific ions to cross the membrane. Pharmacologic blockade of the sarcolemmal ion channels is selective, not only for individual classes of ion channels, but also for specific states of a given type of channel. The basis for this selectivity remains unclear, but may derive from a preferential interaction between a given drug and a specific type of ion-channel protein, or a selective drug action on a structurally specific region of the membrane phospholipid that is in intimate contact with the ion-channel protein.